The GSMA Whitepaper , issued in Feb 2020, provides a good roundup of the various solutions and mitigations being considered for the problem of illicit robocalls. In addition, the ATIS Paper , issued in Feb 2021, also provides an overview of the illicit robocalling problem along with some mitigation techniques and their challenges. The key approaches, according to them, are as follows: 

Blocklisting Robocalls 

The simplest technique to prevent spam or scam robocalls  is the Blocklist. Blocklists can be super-simple – the user simply adds spam/scam numbers immediately after they are received, and if the blocklist is integrated into the phone, new incoming calls can be checked and blocked.  

Crowdsourcing Blocklists 

Crowdsourcing Blocklists are already very popular with consumers – Truecaller, Robokiller, and YouMail are some examples, though all of them now supplement crowdsourcing with other techniques to identify illicit robocalls.  

At its simplest, a crowdsourcing blocklist approach consists of a database and an app. Users install the app and mark every incoming call as spam or not spam. Spam numbers are then kept on the common database, which is looked up for all incoming calls by the app.  

The big challenge with blocklists (crowdsourced or otherwise) is caller-ID spoofing  – illicit robocallers regularly spoof their caller IDs, making it difficult to identify where the incoming call is originating from. Other challenges also exist –  

• Legitimate callers get blacklisted – making it difficult for honest, legitimate businesses to transact business as usual. For example, I often get calls from a food delivery person, and they’re marked as spam. 
• Emergency notification numbers etc. might get blocked. 

Most of the crowdsourcing blocklist providers enable APIs that enterprises can use to get the same value that consumers get.  

STIR/SHAKEN 

Two caller-ID authentication technologies being aggressively promoted and deployed across the US are Secure Telephony Identity Revisited (STIR) and Secure Handling of Asserted information using toKENs (SHAKEN). Pushed by the FTC, service providers across the US are rolling out these standards across their entire network.

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